Sheets or other webs of material are used in a variety of industries and in a variety of ways. These materials can include paper, multi-layer paperboard, and other products manufactured or processed in long webs. As a particular example, long sheets of paper can be manufactured and collected in reels.
It is often necessary or desirable to measure one or more properties of a web of material as the web is being manufactured or processed. Adjustments can then be made to the manufacturing or processing system to ensure that the properties stay within desired ranges. Measurements are often taken using scanners that move scanner heads containing sensors back and forth across the width of the web.
Flat web scanners typically utilize two scanner heads, one above a web and one below the web, that need to be kept in constant alignment with each other in order to minimize errors in process readings. Secondary sensors are often used as diagnostic sensors to ensure head-to-head alignment in three primary directions, namely the X (cross direction), Y (machine direction), and Z (head-to-head gap) directions. Potential interference from the web typically limits the sensor technologies that can be used for measuring the Z gap distance to magnetic, capacitive, or inductive sensing types because opaque web materials often block optical sensors. While relatively low-cost magnetic field orientation sensors exist to allow accurate displacement sensing for X and Y directions, an absolute Z gap measurement has traditionally been limited to the use of expensive inductive coil technologies.